Torque limiting devices have been utilized in various medical applications. For example, torque limiter devices have been used to fasten nuts and bolts that are utilized to secure surgical implements such as orthopedic implants. As such, it is important that these devices enable the user to apply a consistent and exacting amount of torque. The delivery of an exact amount of torque is critical in securing an implant or other surgical implement in the correct position without causing damage to the implant and the patient.
Prior art torque limiting devices are typically constructed with torque gear teeth that tend to bind against the shaft within the torque limiter device. These prior art devices are typically designed such that as the torque gear is rotated within the device, the gear tends to rotate at a slight angle, away from its horizontal position relative to the longitudinal shaft. This misalignment of the torque gear with respect to the longitudinal shaft within these prior torque limiters creates a frictional interference within the device. Such misalignments results in inaccurate torque outputs as well as increased mechanical wear of the device.
In addition, these prior art torque limiter devices are typically constructed with a plurality of intricate and complex components that are intended to fit and work precisely together. However, the complexity and increased number of components generally result in tolerance stack up and other structural misalignments. The cumulative effects of these misalignments further contribute to the inaccuracy of the device as well as to the increased mechanical wear of the device. These inaccuracies are particularly prevalent at low torque ranges, especially when applying a torque at less than 20 inch-lbs. Moreover, it has been known that the act of pushing down on the handle of these prior art devices during their normal standard use could result in the application of an additional 0.5 to 2 inch-lbs. of torque, particularly at these low torque ranges. Since it is critical that the precise amount of torque be applied during surgical procedures, over torquing a fastener could damage the surgical implant and may result in undesirable patient outcomes.
The torque limiter device of the present invention addresses these shortcomings of the prior art. The present invention provides a more accurate device that is designed with a less complicated torque limiting mechanism. The torque limiting mechanism of the present invention minimizes these structural misalignments with the incorporation of a lock bushing and retaining ring within the device. The lock bushing and retaining ring ensure that the shaft of the mechanism rotates true and unobstructed. The simplified novel design further corrects the misalignment issues of the prior art and thus improves the accuracy with which torque is applied. Therefore, the features and structural design of the torque limiting device of the present invention ensure that the proper amount of torque is delivered while securing an implant fastener, thus minimizing possible structural damage to the implant and ensuring patient safety.